1. Field of the Invention
The present invention relates generally to batteries and systems which convert chemical energy into electrical energy by use of a continuous concentration electrochemical cell. More specifically, the present invention relates to an improved method for forming a gas-permeable and ion-permeable membrane for use in an electrode for such systems.
2. Description of the Background Art
U.S. Pat. No. 4,738,904, assigned to the present assignee, discloses a thermoelectrochemical system that functions as a low-temperature power converter in which the electrochemical cell reactants are thermally regenerated at a temperature below about 250.degree. C. This type of thermoelectrochemical system basically includes an electrochemical cell having a cathode compartment and an anode compartment. The two compartments have a common ion-permeable separation wall which allows ions to pass between the two compartments but prevents the passage of gas. A hydrogen ion reacting cathode and a hydrogen ion reacting anode are located within their respective compartments, with the cathode and anode being connectable externally from the system for generation of an electrical voltage and current between the electrodes.
A cathode fluid comprising a chosen Bronsted acid is typically located in the cathode compartment and in contact with the cathode. During one method of operation of the system, hydrogen gas is generated or collected at the cathode and the acid is consumed. The system further includes an anode fluid comprising a chosen Bronsted base which is located in the anode compartment and in contact with the anode. During one method of operation of the system, a cation of the base is generated and the base and hydrogen gas are consumed at the anode. At least one of the components, i.e., acid or base, comprises an organic material.
Because of the gas impermeability of the ion-permeable separation wall, any hydrogen gas generated at the cathode during operation of the system is transferred externally to the anode compartment for consumption at the anode during generation of the electrical current. In addition, during operation of the system, the anions of the acid and/or the cations of the base migrate through the ion-permeable separation wall into the anode or cathode compartment, respectively, where they combine with the cation of the base or the anion of the acid to form the corresponding salt. A feature of this system is that the salt is capable of being thermally decomposed at a temperature below about 250.degree. C. to directly form the acid and base as two decomposition products. These products can be separated to regenerate the acid and base.
A thermal regenerator is provided in these systems for thermally converting the salt directly to the acid and base starting materials, at a temperature below about 250.degree. C. Means for transferring the salt from the anode and/or cathode compartment to the thermal regenerator are also provided. Anode recycle means are provided for transferring the base formed in the thermal regenerator back to the anode compartment to replenish the base consumed during operation of the system. Cathode recycle means are also provided for transferring the acid formed in the thermal regenerator back to the cathode compartment to replenish the acid consumed during operation of the system.
The above-described systems are particularly useful because their relatively low operating temperatures (i.e. below 250.degree. C.) allow them to be used in recovering waste heat in the form of electric power from internal combustion engines, industrial processes, and the like. They can also be used to convert heat from other sources such as solar energy, fossil or nuclear fuel, oil well heads or other geothermal heat sources.
An important consideration in thermoelectrochemical systems, as well as electrochemical systems in general, is the overall efficiency of the system and the useful life. It is therefore desirable to continually search for improvements to such systems in which the performance, efficiency and life of the system are maximized. One such improvement is disclosed in copending patent application Ser. No. 07/132,514, now abandoned filed Dec. 14, 1987 and assigned to the present assignee, and comprises an electrode apparatus for use in electrochemical systems, which includes an anode and cathode between which is located a membrane having both ion permeable and gas-permeable regions to provide transfer of ions and gas between the cathode and anode. The provision of a gas-permeable and ion-permeable separation wall or membrane eliminates the need for externally transferring gases between the two compartments. This elimination of external gas transfer eliminates the need for tubing to accomplish gas transfer, which reduces cost and space requirements. In addition, a system using a gas-permeable and ion-permeable membrane can provide, in some cases, reduced internal cell resistance and increased power as a result of improved gas transfer efficiency between the anode and cathode compartments.
One method for forming such a gas permeable and ion-permeable membrane as disclosed in copending patent application Ser. No. 07/132,514 comprises forming ion-permeable regions in a gas permeable, ion-impermeable material. More specifically, a solution of Nafion was placed on selected regions of a piece of hydrophobic microporous polypropylene. (Nafion is a trademark of E. I. Dupont de Nemours of Wilmington, Del., for a polymer of polytetrafluoroethylene with fluorinated ether side chains terminated with sulfonic acid groups.) Then, the membrane was air dried and cured to form a membrane in which the Nafion impregnated regions were ion-permeable and the unimpregnated regions were gas permeable and ion-impermeable. While this method worked well for its intended purpose, an improvement can be achieved by using the method of present invention.